The need to transport high-bandwidth signals from place to place continues to drive growth in the telecommunications industry. As the demand for high-speed access to data networks, including both the Internet and private networks, continues to evolve, network managers face an increasing need to transport data signals over short distances. For example, in corporate campus environments, it is often necessary to implement high-speed network connections between buildings rapidly and inexpensively, without incurring commitments for long-term service contracts with local telephone companies. Other needs occur in residential areas, including apartment buildings, and even private suburban neighborhoods. Each of these settings requires efficient distribution of high-speed data signals to a number of locations.
An emerging class of products provides a broadband wireless access solution via point-to-point communication links over radio carrier frequencies in the microwave radio band. The telecommunications transport signals may be provided on a wire, but increasingly, these are provided on optical fiber media. An optical to electrical conversion stage is thus first required to convert the baseband digital signal. Next, a microwave frequency radio is needed to up-convert the broadband digital signal to a suitable radio carrier frequency. These up-converters are typically implemented using multi-stage heterodyne receivers and transmitters such that the input baseband signal is modulated and then up-converted to the desired radio frequency. In the case of an OC-3 rate optical transport signal having a bandwidth of 155 MegaHertz (MHz), the input signal may be up converted to an ultimate microwave carrier of, for example, 23 GHz, through several Intermediate Frequency (IF) stages at lower radio frequencies.
Other implementations may use optical technologies to transport the signal over the air. These technologies use optical emitters and detectors operating in the high infrared range. While this approach avoids conversion of the optical input to an electrical signal, it has certain limitations. First, the light wave carrier has a narrow beamwidth, meaning that the transmitter and receiver must be carefully aligned with one another. Light wave carriers are also more susceptible to changes in physical conditions. These changes may be a result of changes in sunlight and shade exposure, or foreign material causing the lenses to become dirty over time. Other problems may occur due to vibrations from nearby passing automobiles and heating ventilating and cooling equipment. Some members of the public are concerned with possible eye damage from high powered lasers.
The present invention is a point-to-point microwave radio link that operates in a Frequency Division Duplex (FDD) mode using separate microwave band radio frequency carriers for each direction. The transmitter uses direct digital modulation to convert an input baseband optical rate signal to the desired microwave frequency carrier. The direct digital modulation is implemented using a Phase Shift Keyed (PSK) scheme. The design may be targeted for operation at unallocated frequencies in the millimeter wave spectrum, such from 40-320 GHz.
In one embodiment, the transmitter is implemented using a direct multiplication followed by a phase shifter. With this arrangement, the transmitter uses a stable voltage controlled oscillator operating in the 10-13 GHz band. The oscillator output is then up-converted to the desired microwave range. For operation in the 40-52 GHz range, this may be a single stage times four (xc3x974) frequency multiplier for operation at a higher range, such as from 81-87 GHz, a second, times two (xc3x972) multiplier may also be employed.
The frequency multiplier output feeds a phase modulator and/or attenuator circuit. In particular, the frequency multiplier output is fed to a phase modulator that deviates the phase of the multiplied output carrier by a desired amount. The phase deviator may be one or more circulators in this preferred embodiment. A bandpass filter and power amplifier may typically be inserted prior to the phase shifter.
The direct digital modulation transmitter may also be implemented using a sub-phase implementation. In this approach, a stable voltage controlled oscillator operating in the 10-13 Giga Hertz (GHz) band is once again used. This oscillator feeds a phase modulator circuit that operates over a narrower phase range than would otherwise typically be used. For example, the phase deviation range is typically only a fraction of the ultimately desired phase deviation range of the output microwave signal. The phase modulator is thus preferably chosen so that it deviates the phase by a desired output amount divided by a particular factor.
That same particular factor is then used by an output frequency multiplier to multiply the phase modulated signal to a higher output carrier frequency. A bandpass filter and power amplifier may then be used to feed a final stage filter prior to forwarding the signal to a transmit antenna.
The phase deviation of the phase modulator in this sub-phase embodiment is preferably chosen to be the reciprocal of the multiplication factor implemented by the frequency multiplier. For example, the phase modulator may implement phase shifts of 0, 22.5, 45, and 67.5 degrees when a frequency multiplier having a multiplication factor of four (4) is applied to an input 10 GHz range VCO signal. After being subjected to the multiplication body output multiplier, the desired output phases of 0, 90, 180, and 270 degrees are provided.
Likewise, in a case where a multiplication factor of 8 is introduced in the output signal processing chain, the phase deviation may be further reduced accordingly. In such an instance, where the output carrier signal generated from the 10 GHz VCO is ultimately multiplied up to a range of 80 GHz, the sub-angle phase deviations implemented by the phase modulator would be 0, 11.5, 22.5, and 33.75 degrees.
If amplitude modulation is also desired, an attenuator may be inserted in-line prior to the phase deviator. This allows multi-level modulation schemes such as QAM to be employed.
The receiver uses a similar but inverse signal chain consisting of a microwave oscillator, frequency multiplier, and bandpass filter. A single down conversion stage is all that is required. By inserting the frequency multiplier between the oscillator and down converter mixer, the local oscillator remains offset by a wide margin from the input RF carrier frequency. This permits the receiver image reject filters to be implemented more easily.
This scheme provides a low cost alternative to traditional approaches, since the base band modem and multiple RF stages are eliminated. Because there are no heterodyne stages, there also are no images of the modulated baseband signals created on either side of the carrier frequency. Thus, image reject filters are not necessary.
Direct digital modulation also only creates modulation artifacts at high multiples of the VCO center frequency. This allows the output bandpass filters to be implemented using inexpensive waveguide technologies that can easily reject the harmonics of the VCO output, as opposed to more stringent filters that might otherwise be required to reject the harmonics of the baseband signal.